In short, it helps you (as you write your e-mail) to avoid words & phrases that might give the wrong tone and might accidentally offend or insult the reader.ToneCheck is offered in the freemium business model: the basic version is free, but if you want more features then you have to pay.

Update 2017-01-22: Added V.QQ.com clip and links.Update 2011-05-12: I'm sorry, I just realized/remembered that of course Youtube is blocked in China. I will research some way to upload this video to a video site accessible in China.